U.S. Pat. No. 3,941,028 discloses a mine roof expansion anchor which comprises a tension bolt extending through a serrated expansible sleeve within a vertical hole in the roof, the inner end of the bolt being threaded into a tapered plug to wedgably expand the serrated sleeve into frictional contact with the inner surface of the hole. Thus, the bond between the bolt and the wall of the hole relies on mechanical friction and is limited by the length of the sleeve.
U.S. Pat. No. 3,731,791 discloses the concept of first inserting into the mine roof hole frangible cartridges containing unmixed quantities of a synthetic resin and a catalyst for quick curing thereof, and then inserting and axially rotating a steel bolt to rupture the cartridges and quickly mix the contents, thus bonding the bolt to the inner surface of the hole along substantially the entire length of the bolt. While this system provides good reinforcing of the mine roof, the steel rods are usually about 7/8" in diameter and up to six feet in length, so that they are heavy and difficult to handle, especially in close and confined spaces. Moreover, there is danger of sparking and causing methane gas explosions during insertion of the steel rod.
Further, such steel rods are dangerous to workers and damaging to mining equipment during the final stage of the mining process when coal pillars are being removed, as the rods are thrown about during the rotary clawing action of the equipment.